Various types of batteries have been developed heretofore, and in every battery, a packaging material is an essential member for sealing battery elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore as battery packages, but in recent years, batteries have been required to be diversified in shape, and desired to be thinner and lighter as performance of, for example, electric cars, hybrid electric cars, personal computers, cameras and mobile phones has been enhanced. However, metallic battery packaging materials that have often been heretofore used have the disadvantage that it is difficult to keep up with diversification in shape, and there is a limit on weight reduction.
Thus, a film-shaped laminate with a base material layer, an adhesive layer, a metal layer and a heat-sealable resin layer laminated in this order has been proposed as a battery packaging material which is easily processed into diversified shapes and is capable of achieving thickness reduction and weight reduction (see, for example, Patent Document 1). The film-shaped battery packaging material is formed in such a manner that a battery element can be sealed by heat-welding the peripheral edge by heat sealing with the heat-sealable resin layers facing each other.
For various packaging materials formed from a laminate as described above, a method for performing printing on a packaging material in the following manner is generally employed: printing is performed on a surface of a base material layer with an ink to form a bar code, a pattern or letters or the like thereon, and an adhesive and a metal foil are laminated on the printed surface of the base material layer (commonly referred to as reverse printing). However, when such a printed surface exists between a base material layer and a metal layer, adhesion between the base material layer and the metal layer is reduced, so that delamination between layers easily occurs. Particularly, for batteries to which battery packaging materials are applied, high safety is required, and therefore a method for performing printing by reverse printing as described above is avoided in battery packaging materials. Therefore, a method has been commonly employed in which a seal provided with printed characters is attached to a surface of a base material layer when printed characters such as a bar code are formed on a battery packaging material.
However, when a seal provided with printed characters is attached to a surface of a base material layer, the thickness and the weight of the battery packaging material increase. Thus, in view of the tendency of further reducing the thickness and the weight of a battery packaging material in recent years, studies have been conducted on a method for performing printing on a battery packaging material by directly printing a surface of a base material layer of a battery packaging material with an ink.
As a method for directly printing a surface of a base material layer of a battery packaging material with an ink, pad printing (also referred to as tampo-printing) is known. The pad printing is the following printing method. First, an ink is fed into a recess portion of a flat plate on which a pattern to be printed is provided by etching. Next, a silicon pad is pressed to the flat plate from above the recess portion to transfer the ink to the silicon pad. Next, the ink transferred to a surface of the silicon pad is transferred to a printed object to apply the ink to the printed object. The pad printing has such an advantage that since an ink is transferred to a printed object using an elastic silicon pad, printing is easily performed on a surface of a battery packaging material after molding, so that printing on a battery is possible after a battery element is sealed with the battery packaging material.
For the base material layer of the battery packaging material, a polyamide resin film such as a nylon film is commonly used, and the polyamide resin film generally contains ethylene bis-oleic acid amide as a water repellant. By including a water repellant, wettability of the molten resin to water is adjusted in production of the film, so that moderate irregularities (surface roughness) can be formed on a surface of the raw film, and even after the raw film is stretched, the irregularity shape remains, so that a polyamide resin film having adjusted surface slippage can be obtained.